The overall objectives of this project are to elucidate regulatory factors in the establishment and maintenance of pregnancy in mammals. The rabbit is used as the experimental model. Specific aims of the proposed research include: 1) Determining the functional importance of prostaglandins and other vasoactive agents in the uterine response to blastocyst attachment at ovo-implantation. 2) Localizing and characterizing the vascular components involved in the early stages of this response at the ultrastructural level and determining the decidual cell precursor in this species. 3) Determining the chemical identity and biological significance of trophoblastic crystalline inclusions in early blastocytes. Chemical and ultrastructural analyses will be employed.